1. Field of the Invention
The present invention relates to a biopolymer composite ion-exchanger with high cesium selectivity that is used to selectively and efficiently separate/recover traces of dissolved cesium from treatment solutions containing various components and resulting from resource recovery, wastewater treatment, chemical analysis, and other procedures; and to a manufacturing method therefor.
2. Description of the Prior Art
Cesium is a metal element of comparatively high average value used in the production of pharmaceuticals, photoelectronic conversion elements, optical crystals, optical glass, and the like. As a natural resource, cesium is widely distributed as an element accompanying other alkali metals, but is not very abundant in the Earth""s crust, where its average content is 3 g/t (xe2x80x9cIppan Chikyu Kagaku (General Geochemistry),xe2x80x9d pp. 54-55, Iwanami Shoten Publishers (1979)). Cesium is mostly produced from pollucite (cesium ore) and is also recovered as a by-product resulting from the production of lithium or potassium from lepidolite or carnallite (xe2x80x9cKisho Kinzoku Data Book (Scarce Metal Data Book),xe2x80x9d Compiled by Metal Mining Agency of Japan (1968)). Even these mineral resources are distributed unevenly across the globe, so selective extraction of trace amounts of cesium dissolved in seawater, geothermal water, and the like has been studied in order to secure this resource (Doctoral Dissertation, Engineering No. 1603, Tohoku University). In addition, the radioactive isotope cesium-137 can be used as a radiation source in medicine and various industries, so selective recovery of this isotope from radioactive liquid waste is an important topic of investigation in terms of volume reduction and efficient use of such radioactive liquid waste (IAEA Technical Rep. Series, No. 353, 1993).
To selectively separate and recover cesium dissolved in trace amounts in aqueous solutions, it has been proposed to use cation-exchange resin techniques (for example, xe2x80x9cIon-Exchangers in Analytical Chemistry. Their Properties and Use in Inorganic Chemistry,xe2x80x9d 14, 170-173, Elsevier (1982)), inorganic ion-sorption techniques (for example, xe2x80x9cIon-Exchangers in Analytical Chemistry. Their Properties and Use in Inorganic Chemistry,xe2x80x9d 14, 173-190, Elsevier (1982)), or solvent extraction techniques (for example, xe2x80x9cSolvent Extraction Manual,xe2x80x9d 202, 403, Technika Publishing House, Kiev (1972)). At the present stage, selective recovery from treatment solutions by inorganic ion exchange is the most viable option in terms of recovery costs (Doctoral Dissertation, Engineering No. 1603, Tohoku University). This is because in comparison with organic ion-exchangers, inorganic ion-exchangers are commonly more selective to specific ions or groups (xe2x80x9cFundamentals of Ion Exchange and Advanced Separation Technology,xe2x80x9d Kodansha Scientific (1991)) and have better heat resistance, radiation resistance, and other physicochemical properties (xe2x80x9cTopics in Inorganic and General Chemistry,xe2x80x9d Elsevier, Amsterdam, 1964)).
The following materials are known as inorganic ion-exchangers highly selective to celium: zeolites (xe2x80x9cZeolite Molecular Sievesxe2x80x94Structure, Chemistry, and Use,xe2x80x9d John Wiley and Sons (1974)), crystalline tetratitanic acid (Nihon Kagaku Kaishi, 10, 1656 (1981)), smectite (Clays Clay Min., 28, 142 (1980)), insoluble ferrocyanides (Proc. of the Symp. on Waste Management, Tucson, 2, 1687 (1993)), ammonium molybdophosphate (Nature, 181, 1530 (1958)), silicon titanates (Ind. Eng. Chem. Res., 33, 2702 (1994)), and the like.
Of these, insoluble ferrocyanides, ammonium molybdophosphate, and silicon titanates in particular have much higher cesium selectivity than do other inorganic ion-exchangers, and are expected to be used as treatment agents for radioactive wastewater, which are typical treatment solutions based on a variety of components (Radiochimica Acta, 40, 49-56 (1986)). In the case of insoluble ferrocyanides, however, it is known that the sorbed cesium is very difficult to elute despite the high cesium selectivity of the material, and significant problems are encountered in terms of recovery and use. Silicon titanates are disadvantageous in that their cesium selectivity diminishes considerably at low pH due to their unstable structure (Ind. Eng. Chem. Res., 33, 2702 (1994)). By contrast, ammonium molybdophosphate has high acid resistance and exceptionally high cesium selectivity, sorbed cesium is completely eluted and recovered into ammonium salts solutions, and the ion-exchanger can be regenerated at the same time (Nature, 181, 1530 (1958)).
At the present stage, therefore, ammonium molybdophosphate is expected to be the most practical inorganic ion-exchanger for use in the separation and recovery of cesium (Radiochimica Acta, 40, 49 (1986)).
However, conventionally synthesized ammonium molybdophosphate is a product with fine powder form (J. Inorg. Nucl. Chem., 27, 227 (1965)), and is thus difficult to handle during solution contact or solid-liquid separation. Compositioning techniques have therefore been studied as a way of using asbestos (J. Inorg. Nucl. Chem., 12, 95 (1959)), silica gels (J. Radioanal. Chem., 21, 381 (1974)), Amberlite XAD-7 (J. Radioanal. Chem., 56, 13 (1980)), polyacrylonitrile (Sep. Sci. Technol., 32, 37 (1997)), titanium phosphate (xe2x80x9cProgress in Ion Exchangexe2x80x94Advances and Applications,xe2x80x9d pp. 289-297, Royal Society of Chemistry (1995)), and the like as matrices in order to improve the handling of such inorganic ion-exchangers.
The aforementioned compositioning techniques are disadvantageous, however, in that they involve performing complex preparation procedures and that the resulting composite lacks reproducibility in terms of ion-exchange characteristics (Radiochimica Acta, 40, 49 (1986)), making these techniques completely unusable on a practical scale. An urgent need therefore exists for developing a new technique for obtaining the inorganic ion-exchangers with greater ease and higher reproducibility.
With the foregoing in view, it is an object of the present invention to provide a manufacturing method of inorganic ion-exchangers, particularly, a novel biopolymer composite ion-exchanger with high cesium selectivity that can be used to sorb/recover cesium from various solutions in an efficient manner, and to provide a highly convenient and reproducible manufacturing method therefor.
The present invention provides an easy-to-handle biopolymer composite ion-exchanger with high cesium selectivity, and a method for manufacturing this ion-exchanger with ease and good reproducibility, and relates to a cesium separation/recovery agent comprising a composite ion-exchanger with high cesium selectivity by employing a calcium alginate gel as a matrix and loading this matrix with an inorganic ion-exchanger; and to a manufacturing method therefor.
The above-described biopolymer composite ion-exchanger with high cesium selectivity is easy to handle and allows solid and liquid fractions to be easily separated, and can thus be used in sorption and separation processes based both on contact filtration and on fixed-phase sorption.
As a result of repeated and thoroughgoing research aimed at attaining the stated object, the inventors perfected the present invention upon discovering that it is possible to obtain an easy-to-handle composite ion-exchanger loaded with inorganic ion-exchangers that is highly selective to cesium and is dispersed in a calcium alginate gel, and to selectively sorb and separate/recover trace amounts of cesium from various solutions having high accompanying salt concentrations with the aid of a column packed with this exchanger.
The present invention comprises the following technical means.
(1) A cesium separation/recovery agent, comprising a composite ion-exchanger with high cesium selectivity, obtained by employing a calcium alginate gel as a matrix and loading this matrix with an inorganic ion-exchanger.
(2) A cesium separation/recovery agent according to (1) above, wherein the inorganic ion-exchanger is one selected from an ammonium molybdophosphate expressed by the general formula (NH4 )3PMo12O40.nH2O (where n is a mol number ranging from about 1 to 4), ammonium tungstophosphorate, and copper (II) potassium hexacyanoferrate (II).
(3) A method for manufacturing a composite ion-exchanger as defined in (1) above, which comprises dispersing an inorganic ion-exchanger powder in a sodium alginate solution to prepare a slurry, bringing a calcium salt solution into contact with the slurry to disperse and load the inorganic ion-exchanger in a calcium alginate gel substrate.
(4) A manufacturing method according to (3) above, wherein the slurry and the calcium salt solution are brought into contact with each other by the dropwise feeding of the first to the second to obtain a granular composite ion-exchanger.
(5) A manufacturing method according to (3) above, wherein the slurry and the calcium salt solution are brought into contact with each other by the extrusion molding of the first in the second to obtain a fibrous composite ion-exchanger.
(6) A manufacturing method according to (3) above, wherein the slurry is formed into a film and then the film is brought into contact with the calcium salt solution to obtain a filmlike composite ion-exchanger.